Thirty-five states now offer early voting, and the share of the vote cast early is certain to rise this year. In 2008, some 40 million people, one-third of the electorate, voted early, Mr. Gronke said.

The early-voting season is just starting. North Carolina, site of the Democratic convention, went first, with mail-in ballots becoming available last Friday.Thus, candidates increasingly treat campaigns as having a series of rolling election days, not just one. Mr. Obama has traveled to Iowa, another important state with early voting, four times in the past month.

His message wasn't subtle. "In Iowa, you don't have to wait until Nov. 6 to vote," Mr. Obama told a crowd in Urbandale recently. "You can be among the first to vote in this election, starting Sept. 27."

The Romney campaign also is pushing supporters to vote early. Republican officials estimate that 75% of the votes in North Carolina, 60% in Florida, 57% in Iowa, and 60% in Nevada—all battleground states—will be cast before Election Day. "There's a plan in place, and we've taken a methodical approach to build our support" through early voting, said Rich Beeson, Mr. Romney's political director.

(...) Traditionally, early voting has been thought to work to Democrats' advantage, because it gives working-class and minority voters—often sympathetic to Democrats, but hard to get to the polls—a broader chance at voting. But Republicans mounted an aggressive early-voting campaign this year during a contentious governor's recall election in Wisconsin.